1. Field of the Invention
The present invention is in the field of string instrument pickup transducers, and the invention relates more particularly to pickups of the type employing piezoelectric transducers that are in direct compressional association with the instrument strings.
2. Description of the Prior Art
Electromechanical transducers or pickups are widely employed in connection with string musical instruments, and particularly in connection with both acoustic and "solid body" types of guitars. Many of these prior art pickups are directly related to the strings of the instrument in an endeavor to reproduce the true sounds of the strings, and these include both wound coil magnetic pickups which are primarily employed in the "solid body" type guitars known as "electric guitars", and piezoelectric crystal pickups which are typically provided in the form of a separate crystal compressed under each string proximate the bridge of the instrument.
A basic defect of all of these prior art pickups that are directly related to the strings of the instrument is that they are not fully responsive to vibratory movements of the strings in all directions transverse to the lengths of the strings, i.e., 360.degree. of transverse string movement. Thus, the conventional wound coil magnetic pickup is responsive primarily only to vertical movements of the strings, and hence is generally non-responsive to the attack which is horizontal in both plectrum and bowed instruments. Similarly, most piezoelectric crystal pickups are also responsive only primarily to vertical movements of the strings, and thereby also substantially fail to respond to the initial attack. Failure of these conventional magnetic and piezoelectric pickups to adequately respond to horizontal or lateral string movements results in an incomplete electrical reproduction of the sonic information generated by the strings, resulting in generally poor fidelity; and in the case of piezoelectric pickups results in such generally low amplitude as to require the use of a preamplifier in advance of the usual amplifier system.
Another problem caused by the vertical sensitivity of conventional piezoelectric crystal pickups is that the pickups also sense vibrational and compressional information in the Helmholtz resonator body of an acoustic instrument, and accordingly the pickups are highly sensitive to various types of "microphonics" or body noises, including acoustic feedback, cross feed from other instruments and other performers' voices, finger and chord noises, and various impacts against the instrument.
Wound coil magnetic pickups have the further disadvantage that they are highly sensitive to stray electrical signals, such as "hum" from stage lighting and other electrical equipment; and many piezoelectric crystal pickups have a similar problem. Wound coil magnetic pickups have the further problem of requiring special strings of magnetic material which are generally inferior in sound to conventional strings.
Examples of typical prior art United States patents disclosing piezoelectric crystal pickups, which employ a separate crystal for each string, are Evans U.S. Pat. No. 3,080,785, Evans U.S. Pat. No. 3,154,701, Scherer U.S. Pat. No. 3,396,284, and Scherer U.S. Pat. No. 3,530,228. The crystals in these patents are responsive primarily to vertical string movements, and horizontal string movements cause a rolling or push-pull action in which one side of each crystal tends to be raised and the other side lowered so as to cancel out horizontal string information.
Various prior attempts have been made to minimize or avoid microphonics or body noises, including isolation of the crystals from the instrument body by vibrationally "dead" supports as in Rickard U.S. Pat. No. 3,712,951, and minimizing the mass of each piezoelectric element as in Evans U.S. Pat. No. 3,073,203. Evans U.S. Pat. No. 3,137,754 seeks to eliminate microphonics by having half of the individual string crystals reversely polarized, but this approach remains sensitive primarily only to vertical string movements and generally insensitive to horizontal string movements. Benioff U.S. Pat. No. 2,222,057 and Scherer U.S. Pat. No. 3,453,902 seek to avoid microphonics by centering each string above a pair of oppositely polarized piezoelectric elements, but this completely eliminates sensitivity of the transducers to vertical string movements and renders them sensitive only to horizontal string movements. Barcus et al U.S. Pat. No. 3,325,580 employs a rocking action of a tall violin bridge in association with a pair of oppositely polarized piezoelectric crystals to avoid microphonics, but as with Benioff and Scherer, this causes cancellation of signals from vertical string vibrations and renders the crystals sensitive primarily only to lateral or transverse vibrations of the strings.
Applicant is aware of no prior art magnetic or piezoelectric transducer directly associated with the strings of a string instrument which has good sensitivity to both vertical and transverse horizontal string movements, and which is therefore fully sensitive to 360.degree. of transverse string movement; while at the same time is substantially completely insensitive to acoustic feedback, body noises, cross feed and other types of microphonics.
Baggs U.S. Pat. No. 4,314,495 encases a series of piezoelectric transducers in a unitary saddle member, and has a form (FIGS. 8 and 9) which he refers to as a "two dimensional" embodiment in which two separate series of the transducers are orthogonally related for sensing "two components of vibratory motion." The trouble with this is that his added set of crystal plates 78 is so arranged as to be able to sense only primarily components that are longitudinal of the string lengths, not lateral, so that they do not cooperate with the principal crystal bar 30 toward 360.degree. of transverse string vibratory movement. Also, while Baggs does alternate the crystal polarities to reduce soundboard noises, he has 2-2/3 times as much crystal area of one polarity than the other in his crystal bar 30, and twice as much in his added set of crystal plates 78, so cancellation of soundboard noises would not be effective.
Another problem in the art of piezoelectric transducers for string instruments is that prior art efforts to modularize or unitize a plurality of piezoelectric crystals into a single structure for convenience of installation and marketing generally resulted in a rigid structure that would not conform to distortions in the bridge and saddle elements of the instrument caused by string tensioning or other factors, so that the pickup tended to not be uniformly responsive to each of the strings, and shimming was sometimes required to improve uniformity. Baggs U.S. Pat. No. 4,314,495 is an example of such prior art modularization. Such prior art efforts toward modularization or unitization of piezoelectric transducers for string instruments had the further problem that the supporting and covering portions thereof were generally sonically incompatible with the materials of which the bridge and saddle elements were made, so that the pickup would tend to introduce a harshness or brittleness into the picked-up sound.
Almost all of the prior art piezoelectric string instrument pickups, particularly those designed for guitars, have followed the Evans approach of a separate crystal (or pair of crystals) for each string. These include Evans U.S. Pat. Nos. 3,073,203, 3,080,785, 3,137,754 and 3,154,701; Sherer U.S. Pat. Nos. 3,396,284, 3,453,920 and 3,530,228; Benioff U.S. Pat. No. 2,222,057; and Rickard U.S. Pat. No. 3,712,951. However, most players have continued to insist upon a conventional, or at least a conventional-appearing, bridge saddle. This is undoubtedly partly because of tradition, but is believed also because the strings must be streched over a narrow, slightly curved surface like that found on the traditional saddle in order to vibrate properly for the proper acoustic effects. Accordingly, this approach of separate crystals for the individual strings has never been truly commercially successful, and is not currently used to any appreciable extent.
Prior art attempts to unitize or modularize the crystals into a saddle member as in Baggs U.S. Pat. No. 4,314,495, or to simply utilize a conventional or other unitary saddle over a series of crystals under the respective strings, have resulted in uneven responses to the strings due to the fact that the ends of the saddle were physically less constrained than the center part, resulting in greater vibratory movement, and hence response, for the end strings than for the center strings. Some help could be provided by shimming, but that is not a satisfactory solution to the problem. Thus, prior to the present invention there has never been a commercially acceptable guitar pickup associated with the bridge in which a traditional or traditional-appearing, saddle was utilized. Also, the larger the area of the crystals, the more difficult it was to get uniformity of response due to manufacting tolerance problems.
Conventional thinking regarding piezoelectric guitar pickups has always been that a relatively large area of piezoelectric transducers was required for good response. Applicant is not aware, however, of any useful prior art consideration of the possible effect of capacitive reactance of the crystals upon the performance characteristics. Applicant has found that typical prior art cumulative crystal transducer areas have a capacitive reactance that has a surprisingly large effect upon the response characteristics of the pickup, as to both amplitude and phase. This is particularly noticeable where only a single string is actuated, yet is adversely effected by the cumulative capacitive reactance of the crystals of all of the strings.